Synthetic fuels processing is a relatively new art. As such, many new and unusual problems have arisen, one of which is the design and fabrication of high performance letdown valves. These letdown valves are used to reduce high pressure reactor effluent slurry to low pressure.
The donor solvent coal liquefaction process, in which these valves are used, produces low sulfur liquid products from bituminous, subbituminous, and other types of coal.
A coal preparation section receives feed coal and crushes it to the desired coal particle size for the liquefaction reaction. A slurry drying section mixes the crushed coal with a hydrogenated recycle solvent stream to form the slurry feed to a liquefaction section. Mixing takes place at approximately 250.degree. F. and moisture that enters with the feed coal is vaporized.
In the liquefaction section, the crushed and dried coal is liquefied in a non-catalytic tubular reaction in the presence of molecular H.sub.2 and the hydrogen donor solvent which was added to the slurry dryer. Reactor operating conditions are approximately 840.degree. F. and 1920 psig.
Effluent from the liquefaction reactor is separated by distillation in a produce recovery section into gas, naphtha, distillates, and a vacuum bottoms slurry. A portion of the distillates serve as feed to a solvent hydrogenation section. In the solvent hydrogenation section, the solvent is catalytically hydrogenated before being recycled for slurrying with the feed coal. The hydrogen donor solvent is a nominal 400/700.degree. F. boiling range material fractionated from the middle boiling range of the hydrogenated liquid product.
The liquefaction reaction section is comprised of a preheat furnace that heats a mixture of feed from the slurry drier and treat gas, the reactors and a separator vessel. A mixture of coal and solvent is pumped to a high pressure level required for the reactors. Hydrogen rich treat gas is mixed with the feed and both pass through the preheat furnace before entering the reactors. Reactor product then enters the separator drum where lighter material is removed in vapor form overhead and heavier liquids exit via drum bottoms. Heavy intermediate product is sent to fractionation facilities for separation into distillates.
The high pressure slurry letdown valve, which is the subject of this invention, controls the level of the liquid within the separator drum.
The throat of this letdown valve is subject to extremely erosive conditions resulting from the three phase flow of coal slurry solids, liquids and gases at extremely high temperatures and pressures. The highly erosive medium can destroy a conventional valve in a matter of hours. The present invention features a new valve design which can withstand the extreme conditions to which it is subjected. The valve of this invention is comprised of an inner valve passageway which has been scientifically engineered to reduce wear to a minimum, and which is easily constructed and maintained.